Optoelectronic semiconductor devices

ABSTRACT

An optoelectronic semiconductor device in the form of an LED comprises a silicon p-n junction having a photoactive region containing beta-iron disilicide (β-FeSi 2 ). The LED produces electroluminescence at a wavelength of about 1.5 μm. Photodetector devices are also described

FIELD OF THE INVENTION

This invention relates to optoelectronic semiconductor devices, forexample photoemitters and photodetectors.

BACKGROUND OF INVENTION

In an optoelectronic semiconductor device there are “photoactive”regions in which either emission or detection of photons can take place.Emission of photons takes place when an applied electrical currentinjects holes and electrons across a junction and the electrons andholes can combine in the photoactive region, the resultant energy beingreleased in the form of photons. Detection of photons takes place whenphotons incident in the photoactive region create electronhole pairs,causing an electrical current to flow.

Silicon has an indirect band gap and this has hindered the developmentof acceptable silicon based photoemitters suitable for use in integratedsilicon optoelectronic applications. Silicon's band gap is also high,hindering development of photodetectors sensitive to wavelengths oflonger than around 1 μm. Optoelectronic devices which are emissive of,or sensitive to, electromagnetic radiation of about 1.5 μm, which is thebasis of optical fibre systems, would be particularly significant incommunications applications and in optical computing systems that areresistant to severe electromagnetic interference (EMI). The devicearchitecture proposed by this invention permits such silicon-basedoptoelectronic devices to be made.

Several different approaches have already been investigated, with a viewto developing a suitable photoemissive device which is capable ofproducing radiation at a wavelength of about 1.5 μm from a silicon-baseddevice.

In one approach SiGe superlattice-based structures have been developedmaking use of zone folding to produce a pseudo direct band gap. Inanother approach, silicon has been doped with erbium which has aninternal transition energy equivalent to 1.5 μm. However, neither ofthese approaches has led to a practical device.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided anoptoelectronic semiconductor device having an architecture wherein ajunction built in an indirect band gap semiconductor substrate hasdirect band gap semiconductor material incorporated to form a distinctphotoactive region.

The introduction of the direct band gap material to form a distinctphotoactive layer related to, but separate from, the junction permits anovel set of semiconductor devices to be created. Semiconductors that donot possess a direct band gap lattice are called “indirect band gap”semiconductors and are generally incapable of efficientelectroluminescence.

The inventors have discovered that provision of a direct band gapsemiconductor material as hereindefined improves the efficiency withwhich photons can be absorbed by, or alternatively emitted from thephotoactive region.

According to another aspect of the invention there is provided anoptoelectronic semiconductor device comprising a junction formed, atleast in part, by a layer of an indirect band gap semiconductormaterial, wherein said layer has a photoactive region containing adirect band gap semiconductor material in which, in operation of thedevice, electron-hole pairs are either created or combined and which hasan energy gap equal to or smaller than the energy gap of the indirectband gap semiconductor material.

Optoelectronic semiconductor devices according to the invention includephotoemitters, e.g. a light emitting diode, and photodetectors, e.g. aphotodiode.

In the case of a photoemitter, charge carriers are transferred acrossthe junction and are injected into the photoactive region where they mayundergo radiative transitions; that is, electrons and holes recombinethere, creating photons whose energy is less than or equal to the bandgap energy of the direct band gap semiconductor material.

In the case of a photodetector, incident photons having an energy equalto or greater than the band gap energy of the direct band gapsemiconductor material create electron-hole pairs in the photoactiveregion giving rise to a photocurrent.

Preferably, said direct band gap semiconductor material has the form ofisolated precipitates or microcrystals. Typically, these are of theorder of fifty to several hundred nanometers so that no significantquantum confinement effects will arise.

Alternatively, though less desirably, the direct band gap semiconductormaterial may form a continuous layer, or a series of continuous layers.

In preferred embodiments of the invention, the direct band gapsemiconductor material is beta-iron disilicide (β-FeSi₂).

Beta iron-disilicide (β-Fe Si₂) a direct band gap semiconductor materialhaving a transition energy corresponding to 1.5 m. Accordingly,embodiments of the invention which incorporate β-FeSi₂ in theirphotoactive regions may find application in optical fibrecommunications. As already explained, it is preferred that the β-FeSi₂be in the form of isolated precipitates or microcrystals; however, acontinuous layer could alternatively be used. Furthermore, the β-FeSi₂may be unalloyed or alloyed, or undoped or doped. β-FeSi₂ alloyed withcobalt germanium, indium or aluminium, for example, has a slightly lowertransition energy than does the undoped material.

In a preferred photoemitter according to the invention, e.g. a lightemitting diode, said junction is a p-n junction formed by a layer ofp-type indirect band gap semiconductor material and a layer of n-typeindirect band gap semiconductor material.

In this specification, the convention is adopted that the layer ofn-type semiconductor material is more heavily doped than is the layer ofp-type semiconductor material. In such circumstances, the photoactiveregion is situated in said layer of p-type semiconductor material sothat under forward-biassed conditions electrons will be injected acrossthe junction and captured by the photoactive region. Alternatively, thelayer of p-type semiconductor material could be more heavily doped thanthe layer of n-type semiconductor material. In this case, thephotoactive region would be situated in said layer of n-typesemiconductor material and holes are injected across the junction fromthe layer of p-type semiconductor material and captured by thephotoactive region.

In the case of a p-n junction emitter, the photoactive region ispreferably situated as close as possible to, but wholly outside, therelatively narrow depletion layer prevailing when a forward bias voltageis being applied across the junction. This configuration is preferred soas to maximise the efficiency with which charge carriers are captured bythe photoactive region where they can undergo radiative transitions.Alternatively, the photoactive region could be spaced apart from thedepletion layer; however, capture of carriers by the photoactive regionwould then be less efficient.

The p-n junction may be a silicon p-n junction; however it is envisagedthat a different homojunction or a heterojunction formed from indirectband gap semiconductor materials could alternatively be used.

In another photoemitter according to the invention, said junction isformed by a layer of indirect band gap semiconductor material and ametallic layer defining a Schottky barrier, and said photoactive regionis situated in said layer of indirect band gap semiconductor material.Under forward biassed conditions carriers are transferred across thejunction and are captured by the photoactive region where they mayundergo radiative transitions.

The photoactive region is preferably situated outside the depletionlayer to maximise the capture efficiency. In this embodiment, theindirect band gap semiconductor material is either n-type material orp-type material, and is preferably, though not necessarily, silicon.

Photodetectors according to the invention include photodiodes, such asavalanche photodiodes and depletion-layer photodiodes.

In the case of an avalanche photodiode, said junction is a p-n junctionformed by a layer of p-type indirect band gap semiconductor material anda layer of n-type indirect band gap semiconductor material, and saidphotoactive region is situated in one or the other of said n-and p-typelayers, outside the depletion layer. Under reverse-biassed conditions,above the breakdown voltage, electrons and holes created by photonsincident in the photoactive region undergo multiplication due to theavalanche process.

The p-n junction may be a silicon p-n junction; however, a differenthomojunction or a heterojunction formed from indirect band gapsemiconductor materials could alternatively be used.

In the case of depletion-layer photodiodes, the photoactive region issituated in the depletion layer. Photons incident in the photoactiveregion are more likely to be absorbed, and thus give rise to a greaterphotocurrent than in prior art.

An example of a depletion-layer photodiode according to the invention isa p-i-n photodiode. In a p-i-n photodiode said junction is formed by alayer of intrinsic indirect band gap semiconductor material sandwichedbetween a layer of p-type indirect band gap semiconductor material and alayer of n-type indirect band gap semiconductor material, and saidphotoactive region is situated in said layer of intrinsic indirect bandgap semiconductor material. Under suitable reverse-biassed conditions,below the breakdown voltage, the photoactive region will be within thedepletion layer in the layer of intrinsic indirect band gapsemiconductor material and electron-hole pairs created by photonsincident in the photoactive region give rise to a photocurrent.

The p-i-n junction may be a silicon p-i-n junction; however,alternatively a different homojunction or a heterojunction formed fromindirect band gap semiconductor materials could be used.

Another embodiment of a depletion-layer photodiode according to theinvention is a Schottky photodiode. In this embodiment, said junction isformed by a layer of indirect band gap semiconductor material and ametallic layer defining a Schottky barrier, and said photoactive layeris situated in said layer of indirect band gap semiconductor material,inside the depletion layer. Electron-hole pairs created by photonsincident in the photoactive region give rise to a photocurrent. Theindirect band gap semiconductor material may be either n-type materialor p-type material, and is preferably, though not necessarily, silicon.

A yet further embodiment of a depletion layer photodiode according tothe invention is a solar cell. In this embodiment, said junction is ap-n junction formed by a layer of p-type indirect band gap semiconductormaterial and a layer of n-type indirect band gap semiconductor material,and said photoactive region is situated within the depletion layer ofthe junction. Electron-hole pairs created by photons incident in thephotoactive region give rise to a photocurrent. The p-n junction of thesolar cell may be a silicon p-n junction; however, alternatively, adifferent homojunction or a hetero junction formed from indirect bandgap semiconductor materials could be used.

Another embodiment of a photodetector according to the invention is abipolar junction transistor—either a n-p-n phototransistor or a p-n-pphototransistor The bipolar junction is formed by layers of p- andn-type indirect band gap semiconductor material forming the transistoremitter, base and collector regions, and said photoactive region issituated in the base region of the junction.

The bipolar junction may be a silicon junction; alternatively, thejunction may be a different homojunction or a hetero junction formedfrom indirect band gap semiconductor materials.

It may be desirable to have one or more barrier layers in the structureof the above-described devices, for example, to control the migration ofimpurities.

In all of the foregoing embodiments of the invention, the direct bandgap semiconductor material is preferably β-FeSi₂ (doped or undoped,alloyed or unalloyed), which is desirably in the form of isolatedprecipitates or microcrystals, though less desirably a continuous layeror series of layers could be used.

According to a yet further aspect of the invention there is provided alaser incorporating a photoemitter as herein defined.

According to another aspect of the invention there is provided a methodfor producing an optoelectronic semiconductor device including ajunction, the method including the steps of forming a layer of anindirect band gap semiconductor material defining part of said junction,and providing in said layer a photoactive region containing a directband gap semiconductor material which has an energy gap equal to orsmaller than that of the indirect band gap semiconductor material and,in which, in operation of the device, electron-hole pairs are eithercreated or combined.

The method may include preforming the junction by a growth techniquesuch as molecular beam epitaxy, for example, and providing said directband gap semiconductor material by an ion implantation process, or viceversa. The direct band gap material may form one or more layers in thephotoactive region. Alternatively, the device could be produced in itsentirety by an ion beam implantation process. Alternatively, the entiredevice could be produced by means of a different growth technique suchas molecular beam epitaxy.

In a particular implementation of the invention, said direct band gapsemiconductor material is provided in said layer as isolatedprecipitates or microcrystals. An example of said direct band gapsemiconductor material is β-FeSi₂.

BRIEF DESCRIPTION OF THE DRAWINGS

Semiconductor devices according to the invention are now described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 shows a schematic transverse cross-sectional view through a lightemitting diode (LED) according to the invention,

FIG. 2 shows plots of electroluminescence intensity produced by thedevice of FIG. 1 as a function of applied forward bias voltage at threedifferent temperatures 80K, 180K and 300K,

FIG. 3 shows the electroluminescence spectrum produced by the device ofFIG. 1 at the temperature 80K,

FIG. 4 shows the variation of electroluminescence intensity of thedevice of FIG. 1 as a function of temperature,

FIG. 5 shows a schematic transverse cross-sectional view through a laserincorporating a photoemitter according to the invention,

FIG. 6 shows a transverse cross-sectional view through anotherphotoemissive device according to the invention, and

FIGS. 7-11 show transverse cross-sectional views through differentphotodetector devices according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the LED comprises a silicon p-n junction 10 formedon a silicon substrate 20. In this example, the silicon substrate 20 isformed from n-type (100) material, and the p-n junction 10 comprises aboron-doped, p-type region 11, 1.0 μm thick grown by molecular beamepitaxy on an antimony-doped, n-type region 12, 0.4 μm thick. The p andn-type regions 11,12 have doping densities of 1×10¹⁷cm⁻³ and 5×10¹⁸cm⁻³respectively, and were grown on the silicon substrate 20 with aresistivity in the range 0.008 to 0.02 Ωcm.

The device is also provided with ohmic contacts 15,16 which are formedrespectively on the outer surface of the p-type region 11 and on theouter surface of substrate 20, whereby to allow a bias voltage V to beapplied across the junction. The contacts 15,16 may conveniently beformed by evaporation and then alloyed onto the respective surfaces; inthis particular example, the contacts are 1 mm in diameter, contact 15,on the p-type region, being formed from Al and contact 16, on thesubstrate, being formed from AuSb eutectic. It will, of course, beappreciated that other suitable contact materials could be used.

Contact 16 on the substrate has a window 17.

The silicon p-n junction 10 has a depletion region 18 (bounded by brokenlines in FIG. 1) at the interface of the p and ntype regions 11,12, andan photoactive region 19 (shown hatched) which is provided in the p-typeregion 11 adjacent to the depletion region 18.

In this embodiment, as will be explained in greater detail hereafter,the photoactive region 19 contains, or consists of, the direct band gapsemiconductor material, beta-iron disilicide (β-FeSi₂)

As already explained, β-FeSi₂ has a band gap corresponding to awavelength of about 1.5 μm. Under forward-biassed conditions of the p-njunction majority carriers (electrons) in the conduction band of then-type region 12 of the junction are injected into the junction andcaptured by the photoactive region 19 where they can undergo radiativetransitions to produce photons at a wavelength of about 1.5 μm. Theresultant electroluminescence is transmitted through the transparentlayers of silicon in the device and exits the device by the window 17 incontact 16.

The photoactive region 19 is so positioned in the p-type region 11 as tobe as close as possible to the depletion layer 18 which prevails when aforward bias voltage is being applied across the junction so as tomaximise the capture of majority carriers by the photoactive region.

In this implementation of the invention, the silicon p-n junction 10 wasinitially grown on substrate 20 by molecular beam epitaxy and thephotoactive region 19 was formed subsequently by ion implantation; thatis, by the implantation of Fe ions.

If relatively high implantation doses are used, region 19 may be formedas a continuous layer of β-FeSi₂. However, it is thought to bepreferable to use a relatively low dose rate (for example, around1×10¹⁶cm⁻²) so as to form isolated precipitates of β-FeSi₂, following anannealing step. A suitable annealing schedule is described by Reeson etal in J Mat Res Soc Proc 316, 433 1994.

The formation of isolated precipitates enables a commercially availableimplanter to be used; also, less processing time is needed and so thecosts in production would be much reduced. In this example, animplantation energy of 950 keV was used whereby to form the β-FeSi₂precipitates immediately adjacent to the depletion region 18, asillustrated in FIG. 1.

Typical precipitate dimensions were of the order of fifty to severalhundred nanometers and so no significant quantum confinement effects areproduced.

Instead of forming the p-n junction 10 and the photoactive region 19 inseparate stages, they could alternatively be formed in a single stageprocess by means of ion beam synthesis.

The LED was found to be emissive of electroluminescence at thewavelength 1.54 μm. As already explained, this wavelength is ofcommercial significance in that it forms the basis of optical fibrecommunications systems.

In order to investigate the properties and characteristics of the LED, anumber of diodes were initially “isolated” by mesa etching down to thesubstrate 20, and the current-voltage (I-V) characteristics of thedevices were measured to investigate diode integrity. Individual diodeswere then separated and mounted in a variable temperature, dynamic,continuous-flow, liquid nitrogen cryostat placed in front of aconventional half-meter spectrometer. A liquid nitrogen cooled germaniump-i-n diode was used to detect the electroluminescence.

Initially, measurements were made at 80K. At this temperature, the onsetof electroluminescence was observed to occur at a forward-biassedvoltage of about 0.8V. The observed electroluminescence intensity as afunction of forward-bias voltage is shown in FIG. 2 for three differentoperating temperatures i.e. 80K, 180K and 300K.

The form of the illustrated curves and the values of the “turn-on”voltages are found to be entirely consistent with conventional injectionacross a forward bias p-n junction.

FIG. 3 shows the electroluminescence spectrum produced by the LED at 80Kwith a forward bias current of 15 mA. The spectrum is observed to peakat 1.54 μm and has a full width at half-height of 50 meV.

FIG. 4 demonstrates that the observed electroluminescence intensitydecreases as a function of increasing temperature, but nevertheless canstill be observed at room temperature (300K). It was noted that the peakin the electroluminescence intensity spectrum shifts slightly towardslower energies as the operating temperature increases, this being acharacteristic of band-edge related emission.

The device was temperature cycled between room temperature and 80K andwas found to operate satisfactorily, in a continuous wave mode, forseveral hundred hours—no significant change or deterioration in theelectroluminescence quality, intensity or operating conditions wereobserved.

It will be understood that although FIG. 1 has been described withreference to a light emitting diode, a p-n structure of this form isalso applicable to other forms of photoemissive devices, e.g. aninjection laser illustrated schematically in FIG. 5.

The laser has a structure similar to that of the LED described withreference to FIG. 1. However, the doping concentration in n-type 12′layer is somewhat higher, being of the order of 10¹⁹ cm³ . An opticalcavity is formed by refecting and partially reflecting elements (R andPR respectively) at each end of the device. In alternative embodiments,the p- and n-type layers 11′,12′, may be interchanged, and thereflecting elements could be positioned above and below the depletionlayer 18′ and the photoactive region 19′ to form a vertical cavitylaser.

FIG. 6 shows a transverse sectional view though a different kind ofphotoemissive device according to the invention which utilises theSchottky effect. Referring to this Figure, the device comprises a layer30 of p-type or n-type silicon and a layer 31 of a metal which togetherform a Schottky barrier. A photoactive region is formed in layer 30 as alayer 32 of the direct band gap semiconductor material, β-FeSi₂. Underforward-biassed conditions electrons are injected across the junctionformed by layers 30 and 31 and are captured by the photoactive regionwhere they undergo radiative transitions. Layer 30 may be formed from ap-type or n-type indirect band gap semiconductor material different fromsilicon.

FIGS. 7 to 11 shows transverse sectional views through differentphotodetector devices according to the invention.

FIG. 7 shows an avalanche photodiode comprising a silicon p-n junctionformed by a layer 40 of p-type silicon and a layer 41 of n-type silicon,and having a photoactive region formed by a layer 42 of β-FeSi₂ situatedin the p-type layer 40, outside the depletion layer D. In an alternativeembodiment, the photoactive region is situated in the n-type layer 41,again outside the depletion layer D. In both configurations, thephotoactive region enhances absorption of light and carrier generationprior to avalanching in the high field depletion layer.

FIG. 8 shows a p-i-n photodiode comprising a layer 50 of intrinsicsilicon sandwiched between a layer 51 of p-type silicon and a layer 52of n-type silicon. In this embodiment, a photoactive region is formed bya layer 53 of β-FeSi₂ situated in the layer 50 of intrinsic silicon.

FIG. 9 shows a Schottky photodiode comprising a layer 60 of p-typesilicon (although n-type silicon could alternatively be used) and ametallic layer 61 which together form a Schottky barrier. In thisembodiment, a photoactive region is formed by a layer 62 of β-FeSi₂situated within the depletion layer D. The photoactive region increasesabsorption and carrier generation in the depletion layer.

FIG. 10 shows a phototransistor comprising a bipolar junction transistorformed by a layer 70 of n-type silicon (defining the emitter region), alayer 72 of n-type silicon (defining the collector region) and a layer71 of p-type silicon (defining the base region). In this embodiment, thephotoactive region is formed by a layer 73 of β-FeSi₂ situated in thebase region. The photoactive region increases absorption and carriergeneration in the base region.

It will be appreciated that a p-n-p bipolar junction may alternativelybe used.

FIG. 11 shows a solar cell comprising a silicon p-n junction formed by alayer 80 of p-type silicon and a layer 81 of ntype silicon. In thisembodiment, the photoactive region is formed by a layer 82 of β-FeSi_(,)situated in the depletion layer D of the junction. The photoactiveregion increases absorption and carrier generation in the depletionlayer.

It will be appreciated that in all the embodiments described withreference to FIGS. 5 to 11, the direct band gap semiconductor material,β-FeSi₂ is preferably in the form of isolated precipitates ormicrocrystals. However, a continuous layer of β-FeSi₂ couldalternatively be used. This material may be formed by processes similarto those described hereinbefore in relation to the fabrication of thedevice of FIG. 1.

Although the preferred direct band gap material is β-FeSi₂ (eitheralloyed or unalloyed, or doped or undoped), it is envisaged thatalternative direct band gap materials could be used; however, it isbelieved that the band gap of these materials should be equal to or lessthan that of the indirect band gap material in which it is situated.

It will be understood from the foregoing that semiconductor devicesaccording to the present invention will find wide applicabilityparticularly, though not exclusively, as optoelectronic sources andoptoelectronic sensors.

What is claimed is:
 1. An optoelectronic semiconductor device comprisinga junction formed, at least in part, from a layer of indirect band gapsemiconductor material, wherein said layer includes a photoactive regionin which, in operation of the device, electron-hole pairs are eithercreated or combined, said photoactive region containing a direct bandgap semiconductor material, being separate from said junction anddefining an energy gap equal to or smaller than the energy gap of theindirect band gap semiconductor material.
 2. An optoelectronicsemiconductor device as claimed in claim 1 in the form of aphotoemitter.
 3. An optoelectronic semiconductor device as claimed inclaim 2 wherein said photoemitter is a light emitting diode (LED).
 4. Anoptoelectronic semiconductor device as claimed in claim 1 in the form ofa photodetector.
 5. An optoelectronic semiconductor device as claimed inclaim 4 wherein the photodetector is a photodiode.
 6. An optoelectronicsemiconductor device as claimed in claim 1 wherein said direct band gapsemiconductor material has the form of isolated precipitates ormicrocrystals.
 7. An optoelectronic semiconductor device as claimed inclaim 6 wherein said isolated precipitates or microcrystals havedimensions of the order of fifty to several hundred nanometres.
 8. Anoptoelectronic semiconductor device as claimed in claim 1 wherein saiddirect band gap semiconductor material forms a continuous layer or aseries of continuous layers.
 9. An optoelectronic semiconductor deviceas claimed in claim 1 wherein said direct band gap semiconductormaterial is beta-iron disilicide (β-FeSi₂).
 10. An optoelectronicsemiconductor device as claimed in claim 9 wherein said β-FeSi₂ isunalloyed or alloyed, or undoped or doped.
 11. An optoelectronicsemiconductor device as claimed in claim 10 wherein said β-FeSi₂ isalloyed with one or more of cobalt, germanium, indium and aluminium. 12.An optoelectronic semiconductor device as claimed in claim 2 whereinsaid junction is a p-n junction formed by a layer of p-type indirectband gap semiconductor material and a layer of n-type indirect band gapsemiconductor material.
 13. An optoelectronic semiconductor device asclaimed in claim 12 wherein said photoactive region is situated as closeas possible to, but wholly outside the depletion layer prevailing when aforward bias is being applied across the junction.
 14. An optoelectronicsemiconductor device as claimed in claim 12 wherein said p-n junction isa silicon p-n junction.
 15. An optoelectronic semiconductor device asclaimed in claim 12 wherein said p-n junction is a non-siliconhomojunction or a heterojunction formed from indirect band gapsemiconductor materials.
 16. An optoelectronic semiconductor device asclaimed in claim 2 wherein said junction is formed by a layer ofindirect band gap semiconductor material and a metallic layer defining aSchottky barrier, and said photoactive region is situated in said layerof indirect band gap semiconductor material.
 17. An optoelectronicsemiconductor device as claimed in claim 16 wherein said photoactiveregion is situated outside the depletion layer.
 18. An optoelectronicsemiconductor device as claimed in claim 16 wherein said indirect bandgap semiconductor material is either n-type material or p-type material.19. An optoelectronic semiconductor device as claimed in claim 18wherein said indirect band gap semiconductor material is silicon.
 20. Anoptoelectronic semiconductor device as claimed in claim 5 wherein saidphotodiode is an avalanche photodiode, said junction being a p-njunction formed by a layer of p-type indirect band gap semiconductormaterial and a layer of n-type indirect band gap semiconductor material,and said photoactive region being situated in one or the other of saidn- and p-type layers outside the depletion layer.
 21. An optoelectronicsemiconductor device as claimed in claim 20 wherein said p-n junction isa silicon p-n junction.
 22. An optoelectronic semiconductor device asclaimed in claim 20 wherein said junction is a non-silicon homojunctionor a heterojunction formed from indirect band gap semiconductormaterials.
 23. An optoelectronic semiconductor device as claimed inclaim 5 wherein said photodiode is a depletion-layer photodiode.
 24. Anoptoelectronic semiconductor device as claimed in claim 23 wherein saiddepletion-layer photodiode is a p-i-n photodiode.
 25. An optoelectronicsemiconductor device as claimed in claim 24 wherein said junction isformed by a layer of intrinsic indirect band gap semiconductor materialsandwiched between a layer of p-type indirect band gap semiconductormaterial and a layer of n-type indirect band gap semiconductor material,and said photoactive region is situated in said layer of intrinsicindirect band gap semiconductor material.
 26. An optoelectronicsemiconductor device as claimed in claim 24 wherein said p-i-n junctionis a silicon p-i-n junction.
 27. An optoelectronic semiconductor deviceas claimed in claim 24 wherein said p-i-n junction is a non-siliconhomojunction or a heterojunction formed from indirect band gapsemiconductor materials.
 28. An optoelectronic semiconductor device asclaimed in claim 23 wherein said depletion-layer photodiode is aSchottky photodiode.
 29. An optoelectronic semiconductor device asclaimed in claim 28 wherein said junction is formed by a layer ofindirect band gap semiconductor material and a metallic layer defining aSchottky barrier, and said photoactive layer is situated in said layerof indirect band gap semiconductor material, inside the depletion layer.30. An optoelectronic semiconductor device as claimed in claim 29wherein said indirect band gap semiconductor material is either n-typematerial or p-type material.
 31. An optoelectronic semiconductor deviceas claimed in claim 30 wherein said indirect band gap semiconductormaterial is silicon.
 32. An optoelectronic semiconductor device asclaimed in claim 23 wherein said depletion-layer photodiode is a solarcell, said junction being a p-n junction formed by a layer of p-typeindirect band gap semiconductor material and a layer of n-type indirectband gap semiconductor material, and said photoactive region is situatedwithin the depletion layer of the junction.
 33. An optoelectronicsemiconductor device as claimed in claim 32 wherein said p-n junction isa silicon p-n junction.
 34. An optoelectronic semiconductor device asclaimed in claim 32 wherein said p-n junction is a non-siliconhomojunction or a heterojunction formed from indirect band gapsemiconductor materials.
 35. An optoelectronic semiconductor device asclaimed in claim 4 wherein said photodetector is a bipolar junctiontransistor comprising layers of p- and -n type indirect band gapsemiconductor material defining the transistor emitter, base andcollector regions, said photoactive region being situated in the baseregion.
 36. An optoelectronic semiconductor device as claimed in claim35 wherein said bipolar junction is a silicon junction.
 37. Anoptoelectronic semiconductor device as claimed in claim 35 wherein saidbipolar junction is a non-silicon homojunction or a heterojunctionformed from indirect band gap semiconductor materials.
 38. Anoptoelectronic semiconductor device as claimed in claim 12 wherein saiddirect band gap semiconductor material is β-FeSi₂.
 39. An optoelectronicsemiconductor device as claimed in claim 38 wherein said β-FeSi₂ isdoped or undoped, or alloyed or unalloyed.
 40. An optoelectronicsemiconductor device as claimed in claim 38 wherein said β-FeSi₂ is inthe form of isolated precipitates or microcrystals.
 41. Anoptoelectronic semiconductor device as claimed in claim 38 wherein saidβ-FeSi₂ is in the form of a continuous layer or series of layers.
 42. Alaser incorporating a photoemitter as claimed in claim
 12. 43. A methodfor producing an optoelectronic semiconductor device including ajunction, the method including the steps of, forming a layer of anindirect band gap semiconductor material defining part of said junction,and providing in said layer a photoactive region, containing a directband gap semiconductor material which has an energy gap equal to orsmaller than that of the indirect band gap semiconductor material,wherein the said photoactive region is separate from said junction andduring operation of the device, electron-hole pairs are either createdor combined in said photoactive region.
 44. A method as claimed in claim43 including preforming the junction by a growth technique such asmolecular beam epitaxy and providing said direct band gap semiconductormaterial by ion implantation.
 45. A method as claimed in claim 43including producing the device in its entirety by an ion beamimplantation process.
 46. A method as claimed in claim 43 includingproducing the device in its entirety by molecular beam epitaxy.